Design & Permitting

(a) Except as provided in division (b)(2) and (c)(1) of 96.08, all erosion and sedimentation control measures, structures, and devices shall be planned, designed, and constructed to provide protection from the calculated maximum peak rate of runoff from the ten-year storm. Runoff rates shall be calculated using the procedures in the latest edition of the United States Department of Agriculture, Natural Resources Conservation Service's "National Engineering Field Handbook", or other acceptable calculation procedures.

 (b)   HQW zones. In high quality water (HQW) zones the following design standards shall apply:

   (1)   Limit on uncovered area. Uncovered areas in HQW zones shall be limited at any time to a maximum total area of twenty acres within the boundaries of the tract. Only the portion of the land-disturbing activity within a HQW zone shall be governed by this section. Larger areas may be uncovered within the boundaries of the tract with the written approval of the Director upon providing engineering justification with a construction sequence that considers phasing, limiting exposure, weekly submitted self-inspection reports, and a more conservative design than the twenty-five year storm.

   (2)   Maximum peak rate of runoff protection. Erosion and sedimentation control measures, structures, and devices within HQW zones shall be planned, designed and constructed to provide protection from the runoff of the twenty-five year storm which produces the maximum peak rate of runoff as calculated according to procedures in the latest edition of the United States Department of Agriculture Natural Resources Conservation Service's "National Engineering Field Handbook" or according to procedures adopted by any other agency of this state or the United States or any generally recognized organization or association.

   (3)   Sediment basin design. Sediment basins within HQW zones shall be designed and constructed according to the following criteria:

      (i)   Use a surface withdrawal mechanism, except when the basin drainage area is less than 1.0 acre;

      (ii)   Have a minimum of 1,800 cubic feet of storage area per acre of disturbed area;

      (iii)   Have a minimum surface area of 325 square feet per cfs of the twenty-five year storm (Q25) peak flow;

      (iv)   Have a minimum dewatering time of 48 hours;

      (v)   Incorporate 3 baffles, unless the basin is less than 20 feet in length, in which case 2 baffles shall be sufficient.

      Upon a written request of the applicant, the Director may allow alternative design and control measures in lieu of meeting the conditions required in divisions (b)(3)(ii) through (b)(3)(v) of this section if the applicant demonstrates that meeting all of those conditions will result in design or operational hardships and that the alternative measures will provide an equal or more effective level of erosion and sediment control on the site. Alternative measures may include quicker application of ground cover, use of sediment flocculants, and use of enhanced ground cover practices.

   (4)   Grade. Newly constructed open channels in HQW zones shall be designed and constructed with side slopes no steeper than two horizontal to one vertical if a vegetative cover is used for stabilization unless soil conditions permit a steeper slope or where the slopes are stabilized by using mechanical devices, structural devices or other forms of ditch liners proven as being effective in restraining accelerated erosion. In any event, the angle for side slopes shall be sufficient to restrain accelerated erosion.

(Ord. No. 21-11-01-001, 11-1-21; Ord. No. 21-12-15-001, 12-15-21; Ord. No. 22-03-16-002, 3-16-22)

(a)   Intent. Stream banks and channels downstream from any land disturbing activity shall be protected from increased degradation by accelerated erosion caused by increased velocity of runoff from the land disturbing activity.

   (b)   Performance standard. Persons shall conduct land-disturbing activity so that the post construction velocity of the 10-year storm runoff in the receiving watercourse to the discharge point does not exceed the greater of:

   (1)   The velocity established by the Maximum Permissible Velocities Table set out within this section; or

   (2)   The velocity of the ten-year storm runoff in the receiving watercourse prior to development.

   If divisions (b)(1) or (b)(2) of this section cannot be met, then the receiving watercourse to and including the discharge point shall be designed and constructed to withstand the expected velocity anywhere the velocity exceeds the "prior to development" velocity by 10%.

   Maximum Permissible Velocities Table

   The following is a table for maximum permissible velocity for storm water discharges in feet per second (F.P.S.) and meters per second (M.P.S.):

   Source - Adapted from recommendations by Special Committee on Irrigation Research, American Society of Civil Engineers, 1926, for channels with straight alignment. For sinuous channels, multiply allowable velocity by 0.95 for slightly sinuous, by 0.9 for moderately sinuous channels, and by 0.8 for highly sinuous channels.

   (c)   Acceptable management measures. Measures applied alone or in combination to satisfy the intent of this section are acceptable if there are no objectionable secondary consequences. The Town recognizes that the management of storm water runoff to minimize or control downstream channel and bank erosion is a developing technology. Innovative techniques and ideas will be considered and may be used when shown to have the potential to produce successful results. Some alternatives, while not exhaustive, are to:

   (1)   Avoid increases in surface runoff volume and velocity by including measures to promote infiltration to compensate for increased runoff from areas rendered impervious;

   (2)   Avoid increases in storm water discharge velocities by using vegetated or roughened swales and waterways in place of closed drains and high velocity paved sections;

   (3)   Provide energy dissipators at outlets of storm drainage facilities to reduce flow velocities to the point of discharge;

   (4)   Protect watercourses subject to accelerated erosion by improving cross sections and/or providing erosion-resistant lining; and

   (5)   Upgrade or replace the receiving device structure or watercourse such that it will receive and conduct the flow to a point where it is no longer subject to degradation from the increased rate of flow or increased velocity.

   (d)   Exceptions. This rule shall not apply where it can be demonstrated to the Town that storm water discharge velocities will not create an erosion problem in the receiving watercourse.

(Ord. No. 21-11-01-001, 11-1-21; Ord. No. 21-12-15-001, 12-15-21; Ord. No. 22-03-16-002, 3-16-22)

If the same person conducts the land-disturbing activity and any related borrow or waste activity, the related borrow or waste activity shall constitute part of the land-disturbing activity, unless the borrow or waste activity is regulated under the Mining Act of 1971, G.S. Ch. 74, Art. 7, or is a landfill regulated by the Division of Waste Management. If the land-disturbing activity and any related borrow or waste activity are not conducted by the same person, they shall be considered by the approving authority as separate land-disturbing activities.

(Ord. No. 21-11-01-001, 11-1-21; Ord. No. 21-12-15-001, 12-15-21)